Electric-discharge machining which is conventionally referred to as EDM has been performed by generating a voltage between an electrode and a workpiece through a dielectric fluid such that a spark momentarily generated removes a portion of the workpiece surface to perform machining. EDM has more recently evolved into electric-discharge texturing (EDT) wherein the surface of a workpiece is textured. For example, rolls used in steel mills to roll steel that is to be painted cannot be too smooth or the paint adhesion will be improper. Texturing of the rolls provides the rolled steel with a textured surface to which paint adheres better. This texturing is performed by an EDT apparatus wherein a head having a bank of electrodes traverses the roll surface as the roll rotates. Actuators move the electrodes towards and away from the roll so as to achieve the correct gap between the electrodes and the roll. The actuators ensure that the electrodes are properly spaced with respect to the roll surface so as to be close enough to generate momentary sparks through dielectric fluid but not too close so as to generate a continuous current flow without any sparks to perform the texturing operation.
A problem with a typical EDT apparatus is that the roll profile deformation at the ends of a roll is different than the roll profile deformation for the central portion of the roll. Specifically, the electrodes in a typical EDT apparatus are spaced apart in the head along a line parallel with the axis of the roll. The head has a given width and traverses across the roll surface from one roll end to the other roll end as the roll rotates. The head reverses direction at each roll end to move the electrodes back toward the other roll end as soon as one of the electrodes lines up with a roll end.
All of the electrodes in the line of electrodes completely pass over the roll surface of the central portion of the roll that is inward from the roll ends by at least the width of the head. However, only some of the electrodes in the line of electrodes pass over the end portions of the roll that are within the head width of the roll ends. For instance, only one electrode passes over the edge of the roll end. Similarly, only two electrodes pass over an adjacent point inward from the edge of the roll end.
With the assumption that all else is equal, the material removal rate is determined by the amount of time a point on the roll surface is being machined. Thus, the central portion of the roll has a higher amount of material removed than the end portions of the roll. As a result, the roll profile deformation is different across the roll.